House construction, how much can we finance?

At the moment, there is nothing. How would the procedure be to find something? Inquire with the city? Which portals should be kept an eye on? Get put on a waiting list somewhere?

I just looked at the city's homepage. It explicitly states that currently no "immediately developable plots" are being sold. You could inquire whether new development areas are being created and, if so, when they will be ready for construction. Otherwise: consider which places around Marburg would be suitable and drive around to see if and where there are free plots, then you could try to get the names and phone numbers of owners through neighbors (of course the same applies in Marburg, but probably more likely in the outskirts) and inquire at the respective municipalities (unless the places already belong to Marburg) whether there are any plots available.

That's exactly how it goes. Although you really have to be quite motivated for the option of driving around and asking neighbors. It’s exhausting and can be frustrating when you only find 1 potentially willing seller out of 10 inquiries. And usually, they don’t want to sell right away but prefer to wait and see if their son will want the property after graduating in 3 months. Or something like that. We are doing this intensively right now and can already tell enough stories..

Actually, I find the reactions here too reserved.

You have a cautious 4500 net. With a net income of 4600, our neighbors just got a 650,000 loan. That gets you quite far.
And if you cautiously include 20,000 from the side business, then you already have 6000 net.
And if the self-employment actually brings in 50k, you even have over 8k net...

So if I were you, I would start looking soon. By the time you find something suitable, hopefully you will have saved more equity with that income, and then you have the best conditions!

With a net income of 4600, our neighbors have just received a loan of 650,000. That's quite a bit already.

How much equity did they have? Any other collateral? Financing for a new build or existing property?

Simply throwing out that you can get a 650k loan with 4600 net income is... well... let's call it daring.

You're right about the rest and it has already been said... actually, you have to start actively looking now if you want a house in a few years. It's best to really get on the municipalities' application lists for planned construction projects. That's not much effort. And you can always still pull out. But at least here there are also municipalities that want a confirmation from the bank of a 520k financing amount with the application for an area that will only be developed in years.

So of course that doesn't mean that the OP will get a 650k loan with their own capital.

It mainly means that currently more is possible than many - myself included - can imagine. I don't know the amount of equity, but surely six figures, as 650k will by far not be enough. Of course, this drastically reduces the risk for the bank and that’s the only reason it worked. It’s about a new build and the plot was already purchased 4 years ago and since then a lot has been paid/saved for the land financing. And as you have already noticed, money usually does come from the parents as well. But I don't know the amount.

In 3 years the OP might have found a good plot/property. By then his girlfriend will know what she wants. By then enough equity should be saved. And by then it will also be a bit clearer what the self-employment yields (for the OP for planning, but also for the bank).

It is meant mainly to say that currently more is possible than some people – myself included – can imagine.

There are plenty of counterexamples. With a 650,000 euro loan, you need about 38 of your income at 2% repayment. At 3%, you pay more than half of the income in your example for the loan. Sounds very unrealistic without a large equity + expensive, paid-off land and expected inheritances in high amounts.